Fix reduce warning for empty arrays

[andreasvirkus]

This fixes an error that gets thrown whenever frappe is fed an empty array for its datasets property.

The error reads:

Uncaught TypeError: Reduce of empty array with no initial value
    at Array.reduce (<anonymous>)
    at getClosestInArray (frappe-charts.esm.js:2671)

Steps To Test:

• Initialise or call update on frappe-charts with datasets: []

TODOs:

• Not sure. Are there any tests that should be modified or created to cover this behaviour?

[coveralls]

Pull Request Test Coverage Report for Build 72

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 38.158%

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Totals
Change from base Build 71:	0.0%
Covered Lines:	29
Relevant Lines:	62

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 72

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 38.158%

---

Totals
Change from base Build 71:	0.0%
Covered Lines:	29
Relevant Lines:	62

---

💛 - Coveralls